Paint of virtually any color can be custom made by beginning with a white base and adding precisely measured amounts of paint colorant of different colors. Sometimes to achieve a desired color it is necessary to dispense very small amounts of paint colorant.
It is important that a paint colorant dispenser have a quick, simple, easily repeatable operation. It is often necessary to prepare many small batches of paint of different colors. If the dispensing operation is not quick and simple then time is wasted and the likelihood of errors increases.
The color of paint can be greatly affected by small variations in the amount of colorant added. Consequently, a paint colorant dispenser must provide precise, repeatable settings.
Prior art paint colorant dispensers typically comprise a reservoir for storing paint colorant and a pump comprising a piston movable within a cylinder and a valve for placing the interior of the cylinder in fluid communication with either a paint colorant reservoir or an outlet. As the piston is raised, paint colorant is drawn into the cylinder through the valve. The valve is then switched to connect the cylinder to the outlet. As the piston is lowered, the paint colorant in the cylinder is expelled through the outlet. The amount of paint colorant dispensed is determined by the stroke of the piston and the bore of the cylinder.
Prior art paint colorant dispensers typically include a stop, adjustable in discrete increments, to limit the travel of the piston. An operator can dispense a selected amount of colorant by fixing the stop in a desired position, raising the piston to the stop, and then lowering the piston.
A difficulty with such paint colorant dispensers is that the stop can be fixed only in discrete positions. If the desired amount of paint colorant is between stop positions then the operator is forced to approximate the desired amount by lifting the piston to a position between stop positions. This method leads to unacceptable errors in the amount of colorant dispensed, especially if the piston is relatively large in diameter.
A previous paint colorant dispenser disclosed in U. S. Pat. No. 4,96.4,534 has two pistons, a small bore piston for dispensing small amounts of colorant, and a larger bore piston for dispensing larger amounts of colorant. Such twin pump dispensers have the disadvantage that the operator may need to manipulate two pistons to dispense a desired amount of colorant. This introduces extra steps into the dispensing process and increases the likelihood of errors. A further disadvantage is that such dispensers have many parts and seals which can lead to increased maintenance costs and reduced reliability.
Some prior art paint colorant dispensers approach the problem of accurately dispensing small amounts of paint colorant by requiring the user to replace a gauge rod in the stop assembly with a separate gauge rod. The separate gauge rod allows the stop to be fixed in a position in which only a small volume of colorant (for example 1/256 fluid ounces) is dispensed. A problem with this approach is that the dispenser should be separately calibrated for use with the separate gauge rod. This increases manufacturing costs. Furthermore, the separate gauge rod is easily lost and installing the separate gauge rod introduces extra steps to the dispensing process.